This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Controlling surface functionalization of 2D materials has emerged as a major requirement in materials chemistry (Mann, J A et al., J. Phys. Chem. Lett. 2013, 4, 2649-2657; MacLeod, J M., et al., Small 2014, 10, 1038-1049). To preserve electronic conjugation within the 2D layer, substrates are often functionalized noncovalently (De Feyter, S. et al., Chem. Soc. Rev. 2003, 32, 139-150; Mali, K S, et al., Nanoscale 2015, 7, 1566-1585; Okawa, Y et al., Nature 2001, 409, 683-684); ligand—substrate interaction can then be used to control electronic structure (Georgakilas, V et al., Chem. Rev. 2012, 112, 6156-6214; Kuila, T, et al, Prog. Mater. Sci. 2012, 57, 1061-1105).
However, utilization of noncovalent monolayers in scalable industrial processes such as solution- and spray-coating elevates the importance of creating robust, solvent-stable films (Bang, J J, et al, J. Am. Chem. Soc. 2016, 138, 4448-4457; Choong, S W, et al., ACS Appl. Mater. Interfaces 2017, 9(22), 19326-19334). A significant body of work in other types of monolayers has demonstrated that desorption of molecules occurs orders of magnitude more rapidly at defects (Doudevski, I. et al, Langmuir 2000, 16, 9381-9384; Love, J C, et al., Chem. Rev. 2005, 105, 1103-1169), suggesting the importance of creating large ordered domains. Here, we report a process for increasing ordered domain areas substantially (over an order of magnitude for the molecules utilized here), using a modified Langmuir-Schaefer (LS) technique that enables in situ thermal control of the substrate during film preparation. We find that monolayers of polymerized diynoic phospholipids prepared in this way are stable toward vigorous washing with both polar and nonpolar solvents, including water, ethanol, tetrahydrofuran (THF), and toluene, suggesting practical utility in applications that require solution processing.
Classical Langmuir-Schaefer (LS) transfer protocols have been utilized since the 1930s to transfer standing phases of amphiphiles to solid substrates (Langmuir, I, et al., J. Am. Chem. Soc. 1938, 60, 1351-1360). In this process, a monolayer of amphiphiles is pre-assembled on an aqueous subphase, and a (usually hydrophobic) substrate is lowered onto the molecular film and withdrawn, transferring molecules to the substrate. Although conventionally utilized to transfer standing phase films (Castellana, E T, et al., Surf. Sci. Rep. 2006, 61, 429-444), LS transfer can also be used to convert standing monolayers on the aqueous subphase into lying-down phases on 2D materials such as highly ordered pyrolytic graphite (HOPG) (Okawa, Y, et al, J. Chem. Phys. 2001, 115, 2317-2322; Giridharagopal, R. et al., J. Phys. Chem. C 2007, 111, 6161-6166).
In classic LS transfer of standing phases, transferred molecules retain their original ordering; thus, transferring from tightly packed source films minimizes defects (Ninks, B P, Adv. Colloid Interface Sci. 1991, 34, 343-432; Honig, E P, et al., J. Colloid Interface Sci. 1973, 45, 92-102). In contrast, in Langmuir-Schaefer transfer involving conversion of standing phases to lying-down phases, each molecule must rotate up to 90° from its initial orientation in the source film to form the horizontally-oriented monolayer (FIG. 1, top left). A LS transfer technique involving conversion is a convenient method to prepare surfaces for scanning probe studies of noncovalent monolayers (e.g. diynoic acids) (Okawa, Y, et al, J. Chem. Phys. 2001, 115, 2317-2322; Giridharagopal, R. et al., J. Phys. Chem. C 2007, 111, 6161-6166; Okawa, Y. et al., Nanoscale 2012, 4, 3013-3028), in which typical domain edge lengths are on the order of ˜100 nm. If adequate control were developed over the conversion process, it could represent a useful means of controlling film structure over length scales from nm to cm.
However, LS transfer involving conversion of standing phases to lying-down phases is both mechanistically more complex and less well understood than classic LS transfer of standing phases; our experience and that of others (Grim, P C M, et al, Angew. Chem. Int. Ed. 1997, 36, 2601-2603) suggests that transfer efficiency and ordering of lying-down phases created using this technique can be quite variable. Further improvement of LS technique is needed to handle transfer involving conversion of standing phases to lying-down phases.
Here, we disclose a process for increasing ordered domain areas substantially (over an order of magnitude for the molecules utilized here), using a modified Langmuir-Schaefer (LS) technique that enables in situ thermal control of the substrate during film preparation. We discovered that monolayers of polymerized diynoic phospholipids prepared in this way are stable toward vigorous washing with both polar and nonpolar solvents (including water, ethanol, tetrahydrofuran (THF), and toluene), suggesting utility in applications that require solution processing.